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Need help identifying missing tubes for mystery amp

K

kanurys

Active Member
Hello,

I'm working on restoring this small power amp. It looks like it came form some sort of turntable console or something since it has plugs to switch the power, an extra a/c plug, and maybe power delivery for an a/c motor.

It appears to be stereo and single ended for each side. The power tube sockets for each channel matches the pinout of an EL84, though I'm not sure what else could plug in there.

-All sockets are 9 pin sockets, 12ax7 sized.

-The transformer windings with no load measure:
Red-Red: 533VAC (hooked up to rectifier pins 1&7), CT to ground
Orange-Orange: 7.13VAC (hooked up to light bulb and external plug)
Green-Green: 7.13VAC (hooked up to all tube heater pins with twisted pair)

-The rectifier socket pinout looks like:
Pin 1&7: transformer red-red
Pin 3: rectifier out to main filter cap
Pin 4&5: filament

-The preamp tube socket looks like:
Pin 1&2: heater filament
Pin 3: N/C?
Pin 4: Cathode A
Pin 5: Grid A
Pin 6: Plate A
Pin 7: Plate B
Pin 8: Grid B
Pin 9: Cathode B

All sections of the main filter cap can are rated for 350VDC

Any help figuring out which tubes to fill this with would be appreciated.
IMG_20210721_114912.jpg IMG_20210721_114938.jpg
 
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6ca4 12ax7 and 6bq5.
A close up of the tube socket opposite the transformer from the bottom side would be helpful but I'd start with those.
6bq5 is the only common power tube that fits the socket. 6ca4 rectifier has the B+ run to it's own pin (cathode sleve) the only one I'm unclear on is the twin triode, which you only have do many options for. 12Au7 doesn't have the gain, 12AX7 does. 12At7, might, but it would be uncommon and probably not ideal as a driver, compared with 12ax7.
We know that the first tube has to have 2 working tubes within it, so that narrows our choices substantially. Cant get 2 tetrodes in a 9 pin bottle (you technically could) so we can assume it's a twin triode. Put a 12Ax7 in, and see what happens...
 
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Single ended 12AX7 into (2) 6BQ5s is most likely. If you can look underneath and compare the pins to where they are connected vs the tubes I listed, it's a start.
 
Thanks for the quick response! Yes, it does make sense that:

The rectifier is a 6ca4
The power tubes are 6bq5 (el84)

You're correct, the dual triode is driving the grids of the power tubes. I just don't know what has that pinout and I do know that it's different from a 12ax7 or similar variant. I think that is what I need help with at this point.
 
trainbuftony said:
6ca4 12ax7 and 6bq5.
A close up of the tube socket opposite the transformer from the bottom side would be helpful but I'd start with those.
6bq5 is the only common power tube that fits the socket. 6ca4 rectifier has the B+ run to it's own pin (cathode sleve) the only one I'm unclear on is the twin triode, which you only have do many options for. 12Au7 doesn't have the gain, 12AX7 does. 12At7, might, but it would be uncommon and probably not ideal as a driver, compared with 12ax7.
We know that the first tube has to have 2 working tubes within it, so that narrows our choices substantially. Cant get 2 tetrodes in a 9 pin bottle (you technically could) so we can assume it's a twin triode. Put a 12Ax7 in, and see what happens...
Click to expand...

Just to be clear, the pinout of that preamp tube is just as I listed it. Here's a pic anyways. Thanks.
IMG_20210721_121945_2.jpg
 
yep, driver has to be a 6EU7. Only twin triode I know of that makes sense with that pinout. 1 and 2 are heater, 3 is usually either grounded or not connected. It doesn't go to anything inside the tube.
 
They are probably right.
6eu7 is the odd duck of the bunch, which did occasionally get used, and was commonly found in repair caddies
 
I sort of wish it was more commonly found, if only so I had a good selection of cheap spares for the one thing I own that uses them.
 
gadget73 said:
I sort of wish it was more commonly found, if only so I had a good selection of cheap spares for the one thing I own that uses them.
Click to expand...

To be honest, if no one came to the rescue (above) I was just going to convert it to a 12ax7 pinout for ease and cost of replacement. I bet you could do this without much effort.
 
kanurys said:
To be honest, if no one came to the rescue (above) I was just going to convert it to a 12ax7 pinout for ease and cost of replacement. I bet you could do this without much effort.
Click to expand...

Not hard, and I've done it before. My one single amp that runs a 6EU7 has it stamped in the chassis so I leave it be. Adapters also exist to let a 12ax7 be used without re-wiring anything.
 
gadget73 said:
Not hard, and I've done it before. My one single amp that runs a 6EU7 has it stamped in the chassis so I leave it be. Adapters also exist to let a 12ax7 be used without re-wiring anything.
Click to expand...
New production 6EU7s are also available ("Tung-Sol") although I expect they are just re-pinned 12AX7s. Don't 6EU7s have spiral heaters or something?

Old production may have gone up in price but they aren't exactly hard to find. Lots of options.
 
Strange tube the 6EU7. I see them mostly in old Gibson amplifiers. Supposed to be low noise, high fidelity, but mostly I find them low gain and drab.

Unless there's an impedance matching issue I'd rewire it to the 12AX7. Just my opinion.
 
Radiomuseum doesn't seem to have it, at least by tube list (searched all combinations of 6EU7, 6CA4, EZ81, 6BQ5, EL84). The OEM part number for the transformers, "1217" doesn't look like a familiar pattern, but date code is 1961, made by Apex Coil, Chicago (EIA code 949). So a midwest company, most likely.
 
6EU7 and 12AX7 are electrically identical, just the pins are shuffled around. No reason they would sound different unless whipped. I think they do have a spiral filament, so I guess its more equivalent to a 7025 or whatever the low noise 12ax7 is.

FlaCharlie said:
Old production may have gone up in price but they aren't exactly hard to find. Lots of options.
Click to expand...

thats the problem, people want actual money for them. I'd like to find them in the dollar tube bin.
 
I can't argue with you @gadget73, (well I could, but I'd lose) but my experience with them has not been good. Even though the Mu is identical they sound more like an AT7 in terms of gain. Perhaps it's just the way the circuits were wired.

I've got a couple kicking around, a Tele smooth plate and an RCA I believe. I wonder why my opinion is so low?
 
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